The invention concerns the area of the biological treatment of effluent for its purification such as, but not exclusively, urban or industrial wastewaters. More precisely, the invention concerns the treatment of sludge derived from the biological treatment of this effluent.
Such methods of biological treatment consist of contacting the effluent to be treated with a biomass which is able to degrade the pollution it contains. The application of such treatments leads to a gradual increase in the quantity of biomass and the need to evacuate excess biomass. This excess biomass is hereinafter called xe2x80x9cexcess sludgexe2x80x9d.
One of the major problems raised by the biological treatment of wastewater relates to the ever increasing quantities of this excess sludge.
Various technical solutions have been imagined to reduce these quantities.
Among these techniques, particular mention may be made of incineration, dewatering, wet oxidation (WO), chemical and biological treatment in order to obtain products that can be given reuse in agriculture, digestion that is to say chiefly methanisation (or anaerobic digestion) or even thermophilic aerobic stabilisation.
Methanisation and thermophilic aerobic digestion can reduce the volume of excess sludge by degrading part of the volatile matter (VM) it contains, up to approximately 50% of the initial volume of the sludge. In addition to the reduction of this volatile matter, these methods lead to the digesting and sanitization of the excess sludge.
It has already been suggested in the prior art to increase the efficacy of sludge digestion by causing it to undergo a preliminary treatment step.
As preliminary treatment it was proposed for example to submit the sludge to mechanical grinding upstream from an anaerobic digester, or even treatment by ultrasound.
With these preliminary treatments it is possible to reduce the residence time of the sludge in the digesters while maintaining a substantial reduction in the volume of volatile matter. However, such pre-treatments, at the most, only lead to removing 60% of the volatile matter contained in the sludge, hence the necessity to remove the residual excess sludge after digestion. Moreover, they require the use of relatively costly equipment.
Another solution, based on the action of ozone was also put forward. For example the process described in European patent application EP-A-0 645 347 suggests applying the action of ozone to the mixed liquor recirculated in the biological basin in order to reduce the production of excess sludge. The main disadvantage of this technique arises from the difficulties inherent in the use of ozone which is a highly oxidizing reagent, and the cost of its use.
The main purpose of the present invention is to put forward a method for treating excess sludge derived from the biological treatment of effluent, with which it is possible to fully eliminate the production of excess sludge.
This objective is achieved through the invention which concerns a method for treating excess sludge derived from at least one facility for the biological treatment of effluent leading to the production of excess sludge, characterized in that it consists of associating at least one digestion step and at least one wet oxidation step of said sludge, said wet oxidation step being conducted at a temperature lying between 50xc2x0 C. and 180xc2x0 C. and at a pressure of between 2 and 40 bar.
It was found, in unexpected manner, that said treatment of excess sludge by wet oxidation under the above-mentioned conditions of temperature and pressure, combined with the biological digestion thereof, made it possible to considerably reduce the volume of residual sludge and could even, when applicable, lead to its total elimination. This result amounts to a considerable advantage compared with the prior art.
It will be noted that in this description by the term xe2x80x9cdigestionxe2x80x9d is meant any method known to persons skilled in the art for degrading sludge with a biomass. This term includes in particular:
anaerobic digestion, also called methanisation, which degrades the organic molecules of the sludge into CO2, CH4 and NH3; anaerobic digestion may be mesophilic, that is to say conducted at a temperature ranging from 30xc2x0 C. to 37xc2x0 C., or thermophilic, that is to say conducted at a higher temperature;
thermophilic aerobic stabilisation conducted at a temperature of between 45xc2x0 C. and 70xc2x0 C., preferably between 50xc2x0 C. and 65xc2x0 C., which consists of biological oxidation made by the injection of air into a stirred tank.
It will also be observed that the conditions of temperature and pressure recommended by the present invention for the wet oxidation step are distinctly less harsh that the usual conditions for WO for which the temperatures used are generally between 180xc2x0 and 300xc2x0 C. with pressures ranging up to 120 bar. With the conditions of the invention, it is possible to inactivate the biomass so that it can be partly oxidized and solubilized, but in unexpected manner without causing the formation of insoluble mineral residues (oxides, carbonates, sulfates, etc.. ) which could not be removed during the digestion step and which would hinder the conducting of this step and therefore limit the possibilities of reducing the quantity of sludge to be evacuated.
According to one preferred variant of the invention, the method consists of subjecting said sludge to digestion, wet oxidizing the digested sludge, and recirculating said oxidized sludge towards said digestion step. A quantity of biomass in equal or higher quantity to the excess sludge from a conventional biological waste treatment process could be recirculated in said oxidation unit.
According to another variant, the method of the invention may also consist of wet oxidizing the excess sludge and then causing it to undergo digestion.
Also, according to another variant, the method comprises an additional step to thicken the sludge conducted before or after said wet oxidation step.
The wet oxidation step recommended above may be applied for a treatment time that varies in particular in relation to the effluent to be treated. However, according to one preferred variant, this step is preferably conducted for a time period of between 10 mn and 180 mn.
In order to further reinforce the efficacy of this step, it may also be preferable to provide for prior pH adjustment of the sludge derived from the biological treatment facility, to a value varying between 2 and 11 as required.
For the same purpose of increasing the efficacy of the wet oxidation step under the present invention, this step may advantageously be conducted using an oxidant/chemical oxygen demand (COD) ratio of the excess non-treated sludge that lies between approximately 0.2 and 1.5.
The oxidizing agent used during the wet oxidation step may be air, oxygen, oxygen-enriched air, hydrogen peroxide either alone or in combination with one of the three previously cited compounds.
According to one variant of the method, a catalyst may advantageously be used for the wet oxidation step to increase the efficacy of COD reduction, COD solubilisation and the reduction of suspended solids (SS) in the sludge to be treated. This catalyst may be homogeneous or heterogeneous.
Preferably, this catalyst is a metal chosen from the group made up of manganese, iron, cobalt, nickel, copper, zinc and the mixtures and compounds of one or more of the latter. In best preferred manner, this homogeneous catalyst is a soluble compound of copper and/or zinc.
According to one advantageous variant of the method, ammonia-containing nitrogen may be removed during the oxidizing treatment of the sludge in catalytic manner by using a heterogeneous catalyst in the oxidation unit positioned above the interface measurement means 7 to measure the concentration of dissolved oxygen in the effluent and means 8 to adjust the operation of the aerating means 3 in relation to the results of the measurements made by said measurement means 7.